Installing Cherokee 140 propellor on O320-E2A.

[AeroLudite]

Ever since my previous annual in ‘22, my plane has a “vibration”. That’s obviously related to the prop.
At the annual, the prop was removed for the crank shaft AD inspection. Previously the plane was remarkably smooth running. Now, NOT.
PREVIOUSLY, the prop would stop at the 10 o’clock position as viewed from in front of the plane. Now, it stops at the 2 o’clock position. My mechanic IA is adamant that this is correct. However, at a 100hr inspection, another mechanic at my request reinstalled it to stop at 10 o’clock. It ran smoother, again. But, at most recent annual the prop was reinstalled to stop at 2 o’clock and vibration returned. It’s enough that’s it’s loosening interior screws!
Which is correct?

 

[RyanB]

Prop installation shouldn’t affect the smoothness of the engine. That said, there are indexing marks for installation that should be followed. Did they?

 

[Dan Thomas]

The Cherokee manual does not state a position relative to crankshaft position.



The '77-'86 172 service manual, like all Cessna manuals, give the position:



That Cessna advice places the prop in a position that will end up with a blade at the 10 o'clock position. That is where the crankshaft is in a position where one piston is halfway down its power stroke and another is halfway up its compression stroke, and it's where the prop is in the most handy and safe place for hand-propping. It also minimizes vibration by placing the propeller's mass in a roughly level position when the engine fires, damping the left-right rocking motion of the engine every time it fires. The mechanic that installs it at the 10 o'clock position has a little more education on this.

There are other possible factors. Was that engine ever dynamically balanced? If so, there may be some weights bolted to the spinner backplate or ring gear, and they are in a position to counter propeller imbalance. Getting the prop in the wrong clock position, or installing it 180° off from where the balance was done, will bring on vibration.

The balance weights are normally some washers, sometimes a mix of fender and standard AN washers.

Propeller dynamics are far more complicated than many pilots realize. The Cessna 185 has a three-blade prop. Some of those airplanes also had the alternator mounted on rubber cushions, with the adjustment arm also cushioned, perhaps to reduce engine vibration damage to the alternator. The alternator is on the left aft side of the engine, far from the prop. But that alternator would wiggle around too much sometimes and that alternator drive belt would be thrown off the pulleys, and the fix was to take the prop off and reinstall it 180° off from where it had been. That stopped the vibration, usually. A lot of those cushioned mounts have been replaced with the rigid mounting that causes far less trouble, including grounding issues that required an AD to correct.

Now, why would changing the prop position fix that? Well, I suspect that a downgoing blade in the climb was pulling the engine's nose to the left every time a blade was horizontal on the right, and perhaps that vibration, combined with the usual left-right rocking motion caused by the movement of pistons that are not directly opposite each other, was doing it. Reclocking the prop might have broken up that relationship.

 

[AeroLudite]

Yes, BUT
I Went around with the mechanic on a Cessna 150 I owned. Turns out, he was referring to the Cessna manual and the picture in the manual is turned BACKWARDS! (Mirror image reversal). He had followed photo to confirm installation. Following text placed prop opposite photo… text is correct.
One installation has prop leading combustion stroke. Other following combustion stroke, so, IT DOES matter…

 

[Magman]

It can also be that one blade is heavier than the other.

You might try turning it 180 degrees.

Tracking is an essential part of Prop Installation to assure all blades are in

the same plane of rotation. Easier with 1 spark plug out.

Cars have “ Harmonic Balancers” on the Crankshaft to absorb vibration.

Propellers serve the same function on aircraft. There is a series of pulses

with a piston engine rather than smooth application of power with a

turbine,

In addition to all this the prop must keep the pilot cool.

If it stops turning the pilot starts sweating!

 

[biplanebob]

Propeller Vibration Troubleshooting check list from Sensenich: